As a user, Primal is clearly the frontrunner. However, some of these clients are NOT functional, as of recently are slacking, and are unusable. I agree with @fiatjaf post yesterday re wasting Jack's money, and calling some developers "opportunists."  I would highly recommend initially screening these developers for actual programming/coding skills and view their past work.  And just like any other job, these developers FIRST need to work, perform, and deliver, before they get compensated. And not the other way around. Perhaps compensating developers via a "piece work" system is the optimal way to produce tangible results. Otherwise, these developers take the bounty and run. 
As an HR Analyst, and having my own corporations, I would NOT continue to compensate several of these clients, because they are underperforming and add NO value. 

https://primal.net/e/note1hm44ma069v53ckju985up5egr8ft2w0mm7wurysrvgsuduauxdmq42a55m 

 cc:  @jack